The present invention relates to fluid heater apparatus, and more particularly to an improved fluid heater wherein compensation in the heating temperature may be made under both static and dynamic fluid flow conditions, and through monitoring of the ambient temperature in which the apparatus operates. The invention is particularly directed to the heating of paints, lacquers, varnishes, and other single and plural component materials.
In industrial coating operations it is extremely important to control the viscosity of the coating materials, particular when those coating materials are applied by some sort of spraying apparatus wherein it is desirable to create an atomized spray of the coating materials for relatively even application upon a surface to be coated. While there are a number of ways to control fluid viscosity, such as by material and solvent selection, it is particularly convenient to control such viscosity through the control of the temperature of the material. When such materials are applied by means of a spraying apparatus it is important to control the viscosity over fairly narrow ranges. For example, it has been found that viscosity changes exceeding plus or minus eight percent cause a noticeable change in the spray pattern of the fluid as it is being applied, and it has therefore been desirable to control the temperature of the fluid within plus or minus 3.degree. F. The problem is complicated by the fact that spraying devices typically are operated intermittently, resulting in both static and dynamic flow conditions in the fluid being fed into the spray device. Thus a fluid heater must have the capability of precise temperature regulation, so as to decrease the amount of heat applied to the fluid under static conditions, and yet increase the amount of heat applied as soon as the liquid fluid flow conditions become dynamic. Further, it has been found that the ambient temperature of the work place affects the quality of the spray pattern if such ambient temperature varies over significant ranges. Since temperature control in industrial plants is typically poorly regulated, it is not unusual for the work place temperature to vary over as much as 30.degree. F.-40.degree. F. over the time span of a single work shift. When such equipment is used outdoors the problem becomes more severe, because ambient temperature ranges may be even broader.
In the past it has been common practice when relatively constant temperature operating conditions are desired to utilize a high mass fluid heater which may be slowly heated to a desirable temperature, and once heated is relatively insensitive to fluid flow conditions. However, such systems are usually operated at a temperature well above the desired fluid temperature, in order to impart enough heat into the fluid during dynamic flow conditions. This creates an excessive temperature problem under intermittent operating conditions, particularly when the spray device is turned off for a period of time, for then the fluid in the heater will rise to the temperature of the mass being heated. Attempts have been made to compensate for this problem by sensing the fluid temperature rather than the temperature of the mass being heated, and shutting off the heating power to the mass as the fluid temperature reaches the desired level. Similarly, as the fluid temperature drops below the desired level power is applied to heat the mass to return the fluid temperature to the desired level. The problem with this approach lies in the relatively long time it takes to heat and/or cool the large mass through which the fluid passes, resulting in fluid temperature swings both above and below the desired operating range.
It is thus desirable to use a low mass heating element with provision for controlling the temperature of the heating element very rapidly to accommodate fluid flow conditions. Since it is important to obtain a close control over fluid temperature, and since this temperature is affected to a significant degree by ambient temperature, it is also important to sense ambient temperature and use this condition as a further regulation on heating element temperature. Further, it is desirable to utilize a low mass heating element having the capability of controlling evenly the temperature of all of the fluid contained therein or flowing therethrough.